Channel estimation is a key technology for Wireless Optical Communication (WOC) systems. Based on the Training Sequence (TS), this paper develops three Frequency Domain (FD) channel estimation approaches for diffuse wireless optical channels. Considering the propagation property of light and the characteristics of optical modulation, this paper establishes a link model for the indoor WOC systems. Using the established link model, three FD channel estimation methods, such as the LS method, the Minimum Mean Square Error (MMSE) method and the ORL-MMSE method are proposed. The computational complexity analysis gives time complexities of the three channel estimation methods in FD. This paper also evaluates the performance of the proposed three methods by computer simulation, measuring in terms of MSE and BER, respectively.